1. Field of the Invention
For spraying e.g. powdery material in a molten state onto a substrate surface, plasma spray apparatuses are well known in the art which make use of an indirect plasmatron, i.e. an apparatus for creating a plasma with a plasma torch escaping from a nozzle-like element which plasma torch is electrically not current conductive. Usually, the plasma is created by means of a torch and guided through a plasma channel to an outlet nozzle. Thereby, an important difference exists between an apparatus with a short plasma torch and an apparatus with an elongated plasma torch.
2. Prior Art
In a major portion of all plasma spray apparatuses which are commercially used in these days, the plasma torch is created by means of a high current arc discharge between a pin-shaped cathode member and a hollow cylinder anode member. Thereby, the coating material which has to be molten and axially accelerated, e.g. powdery material like metallic or ceramic powder, is introduced into the plasma torch and thereby molten. Many of these plasma spray apparatuses incorporating an indirect plasmatron have the disadvantage that the free plasma torch is not sufficiently stable as far as heat intensity and the position of its radial temperature profile. The result is that the powdery material fed into the plasma torch is thermically unevenly treated; thus, the coatings created with the sprayed material do not have the desired finish.
The reason for this irregularity of the plasma torch in those plasma spray apparatuses may be seen, on the one hand, in the instability of the plasma torch which can have many different causes. Thereby, an important role plays the fact that the foot of the electric arc travels along the extension of the electrodes under certain circumstances. On the other hand, in connection with this traveling of the foot of the electric arc, the thus resulting asymmetric shape of the electric arc with respect to the central longitudinal axis of the plasmatron results in an uneven thermal treatment of the powdery material.
Particularly pronounced are foot travel effects of the electric arc in plasmatrons which operate with a short electric arc, whereby a pin-shaped cathode penetrates the interior of a one-part, nozzle-like anode (cf. German Utility Model No. 1,932,150) because with anode nozzles having an axial extension not only axial but also peripheral travel effects of the foot of the electric arc can occur. At least axial foot travel effects are to be expected in a similar plasma spray apparatus disclosed in German Publication No. 3,312,232 which has not one single, but several cathodes.
Principally, an axial foot travel effect arises due to the fact that an electric arc between a cathode member and a nozzle-shaped anode member is axially stretched, under the influence of the plasma flow, from the cathode member to a point on the anode member which has the greatest distance from the cathode member. Then, the electric arc breaks away from the above mentioned far point of the anode member and attaches again at a point of the anode member which is next to the cathode member. Experience has shown that this phenomena is more or less periodically repeated with a frequency in the region of several kcps. The voltage variations coupled with these variations in length of the electric arc result in severe energy variations (up to .+-.30%) and, thus in corresponding variations of the intensity of the free plasma torch. Thereby, the powdery material fed into the plasma torch is irregularly treated.
The asymmetric shape of the electric arc has as a result that also the radial temperature profile of the free plasma torch is asymmetric; i.e., the hot central region of the plasma torch is subjected to a certain deviation from the central longitudinal axis of the plasmatron. This deviation is even increased by the fact that the plasma flowing out of the anode nozzle is further heated at the foot of the electric arc, i.e. at an eccentrically located position of the plasmatron. Particularly aggravating is such a deviation of the plasma torch in combination with a peripheral foot traveling of the electric arc. Thereby, a sort of precession motion of the plasma torch is created which usually has an irregular course and results in an even worse treatment of the powdery material if the powdery material is externally fed from a stationary feeding means.
Somewhat better results in these respects can be achieved with a plasma spray apparatus the plasmatron of which operates with a long electric arc, e.g. as disclosed in the European Publication No. 0,249,238 A2. This plasma spray apparatus comprises a plasma channel comprising an annular anode member and a plurality of annular neutrode members which are electrically insulated from each other. By means of this cascade-like design of the plasma channel with its plurality of neutrodes placed in front of the anode member, an axial foot traveling of the electric arc at the anode-sided end thereof is avoided. However, in such a plasmatron, there is a pronounced peripheral foot traveling of the electric arc along the annular anode member if the electric arc originates from a single cathode member, as disclosed e.g. in the European Publication No. 0,249,238 A2. In this respect, the conditions are similar to the ones described herein before in connection with a plasmatron operating with a short electric arc. Thus, also in this case, an uneven thermal treatment of the laterally fed powdery material occurs.